This invention relates to the CO2 gas laser and in particular to the high pulse repetition rate, high pressure transverse discharge type.
The CO2 gas laser requires a plasma glow discharge to excite the lasing species which then generates laser radiation by stimulated emission. In addition to providing the laser excitation energy, the glow discharge also acts negatively to dissociate the CO2 gas into its constituent CO and O2 parts. These constituents are essentially chemically stable and by themselves do not reform into CO2 over timescales of interest in normal application. The result is a depletion of the CO2 lasing species and a buildup of CO and O2 in the laser gas which leads to a loss of laser power and a tendency to discharge arcing. Discharge arcing is the phenomenon in which the glow discharge collapses into hot, narrow channels seriously degrading laser output energy and beam uniformity. In general, there are two types of CO2 laser, those that operate at relatively low gas pressure on the order of 20 Torr and those that operate at much higher pressures on the order of one atmosphere or 760 Torr. The discharge in a low pressure CO2 laser is normally of the continuous wave (cw) type; whereas, the high pressure discharge is of the Transverse Electric Atmospheric (TEA) short pulse type. Low pressure glow discharges induce a relatively low rate of CO2 dissociation; and high pressure discharges, especially at high pulse repetition frequencies, lead to rapid loss of CO2 and generation of O2. Even relatively low levels of O2 on the order of about 1% can lead to arcing in high pressure discharges.
In order to control the loss of CO2 in low pressure lasers, a continually fresh gas flow into the laser and exhaust of depleted gas can be used in some applications in which the presence of a large feed gas reservoir is acceptable. Another approach for low pressure sealed CO2 lasers is gas catalysis in which certain gas additives can be used to promote CO2 regeneration, but regeneration by this method operates at a relatively low rate. In the case of the high pressure TEA CO2 laser, the method of fresh gas replacement would require prohibitively large amounts of gas; and the regeneration rates by gas catalysis are too low, especially for compact lasers with small gas volumes and high specific energy discharges.
Alternatively for the high pressure TEA CO2 laser, an effective method to control gas decomposition and maintain discharge stability is with a highly active, solid state discrete CO+1/2O2 to CO2 regeneration catalyst which can be implemented in a completely sealed laser system with a single charge of gas. Such catalyst is also applicable to the low pressure CO2 laser for situations in which continual gas flow replenishment is not feasible or where gas catalysis becomes unreliable over very long operating periods. For both low and high pressure lasers, a discrete solid state regeneration catalyst in a sealed laser gas vessel would be desirable when using a gas mixture with various expensive rare isotopes such as 13C16O2 which are often employed to generate additional laser wavelengths beyond those available from the commonly occurring isotope 12C16O2.
The discrete solid state catalyst can take several different forms, including small spheres or rods of approximately 3-5 mm diameter and disks of approximately 15 mm diameter. The small rods and spheres are suitable for high surface area pebble bed catalyst modules, but they induce very high pressure drops in high flow rate gases typical of high power laser systems; whereas, the disks can be placed separately in the flow stream for low flow impedance but at the expense of reduced total catalyst surface area. For these catalysts, it is the combination of intrinsic catalyst chemical reactivity in combination with surface area that determines net total catalyst effectiveness. U.S. Pat. Nos. 4,943,550 and 5,017,357 teach how such catalysts are fabricated by applying thin layers of platinum compounds to ceramic substrates. Several basic catalyst types have been described and compared in Lewis, “Catalyst Selection for a Rep-Pulsed High Power Self-Sustained Discharge CO2 Laser”, SPIE 2702, 385 (1996). Whatever the catalyst geometry, they are all coated with a thin layer of active catalytic material which is loosely bound to the underlying inert substrate, and as a consequence they all liberate dust when handled or rubbed together. The dust particles eventually find their way to the laser vessel windows where they deposit forming sites for ablation in the highly intense intracavity laser resonator optical beam. The ablation sites cause damage to the window optical surfaces leading to increasing disruption of the laser optical mode and decrease in output power over time. Optical damage is a major laser lifetime limiting effect.
The effectiveness of discrete catalysts depends on two other important factors, their temperature and the flow rate of gas through them. Catalytic activity itself increases markedly with increasing temperature, roughly doubling with every 50 C rise in temperature. The rate of CO+1/2O2 to CO2 regeneration depends upon the rate at which decomposed gas is recycled through the section of the laser containing the catalyst. These two factors plus the acceptable catalyst volume that can be set aside in the laser design are balanced against the CO2 decomposition rate which is dependent on the strength of the discharge, pulse energy and pulse repetition frequency in the case of the CO2 TEA type.
The CO2 TEA laser is characterized by fast transverse gas flow across two long parallel electrodes between which the short pulse glow discharge is initiated. The gas velocity is about 6 m/s to 12 m/s for 1 cm wide electrodes with a discharge pulsing at a 200-400 Hz rate, which repetition rates are required for many applications. The gas flows in a recirculating pattern, alternately passing across the electrodes and through a heat exchanger. The motive force for the gas flow is provided by a fan extending the full length of the electrodes. This basic geometry is described in U.S. Pat. Nos. 4,099,143 and 4,686,680. Incorporation of a catalyst module in the basic TEA laser geometry can be achieved by placing the module within the main flow loop or in an auxiliary gas flow arm outside of it. Placed within the main gas flow, the catalyst module will induce an impedance to the main flow which will become more severe as the amount and density of catalyst is increased. For a module placed within the main gas flow stream, it is also not desireable to heat the catalyst to increase activity because such heating will elevate the average temperature of the surrounding lasing gas thereby reducing laser gain and efficiency. A rise in gas temperature from 21 to 30 C causes roughly a factor of two reduction in output energy. One approach to reducing the effective gas flow impedance imposed by the catalyst module is to place it adjacent to the walls of the main gas flow stream and direct only a portion of the main gas stream through the catalyst; but the catalyst module then becomes much less effective, as most of the gas bypasses it, and the result is a significant reduction of the maximum allowable discharge pulse repetition frequency. This side-wall implementation has been described in D. S. Stark, et al. “A sealed 100-Hz CO2 TEA laser using high CO2 concentrations and ambient temperature catalyst”, J. Phys. E: Sci. Instrum., Vol. 16, 158-161 (1983).
The implementation of a catalyst module in an external flow loop is described in Willis and Purdon, “Catalytic Control of the Gas Chemistry of Sealed TEA CO2 Lasers”, J. Appl. Phys. 50 (4) April 1979 and Willis, et al., “Use of 13CO2 in high-power pulsed TEA lasers”, Rev. Sci. Instrum., 50, 1141-1143 (1979). These publications describe catalyst modules with no particulate control and with low flow rates and low net recombination activity that can support relatively low 2.5 Hz pulse repetition frequencies. The publication by Lewis makes mention of efforts to develop an auxiliary flow loop catalyst module; but no details, diagrams or test results are given, although a maximum pulse repetition frequency of 50 Hz is indicated in a data plot. The auxiliary flow loop catalyst configuration can also be utilized with low pressure lasers.